You can't really make that either though. Facebook itself can see everything. As soon as you federate that all the admins can see everything and nothing is private.

You could try and encrypt it down to the user level but a person added to a group would only see the content added from that point forward (the ability to decrypt would be defined at the time of the post, and new people could never read it).

The only way around that would be centralised key management which defeats the whole point.

This is one of those problems where it's probably better to just use Facebook.